Alkaline secondary batteries represented by nickel-hydrogen batteries and nonaqueous electrolyte secondary batteries represented by lithium ion batteries are widely used as power sources for cellular phones, portable computers, portable music players and other portable electronic devices. As curbs on emissions of carbon dioxide and other substances have been strengthened against a background of growing movements for environmental protection, in the automobile world there is now vigorous development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) alongside vehicles using fossil fuels such as gasoline, diesel oil and natural gas. In addition, the soaring prices of fossil fuels in recent years have acted to spur on the development of EVs, HEVs and the like.
These nonaqueous electrolyte secondary batteries include cylindrical and prismatic batteries. Since a number of batteries are serially and parallely coupled especially for use in EVs and HEVs, prismatic batteries with favorable spacing efficiency are required. As for such a prismatic battery, for example, a prismatic nonaqueous electrolyte secondary battery is fabricated as follows.
Negative electrode plates are fabricated by applying a negative electrode binder containing a negative electrode active material on both surfaces of a negative electrode substrate (collector) constituted of copper foil or the like having an elongated sheet-like shape. Also, positive electrode plates are fabricated by applying a positive electrode binder containing a positive electrode active material on both surfaces of a positive electrode substrate constituted of aluminum foil or the like having an elongated sheet-like shape. Between a negative electrode plate and a positive electrode plate, a separator made of a microporous polyethylene film or the like is interposed. With the negative electrode plate and the positive electrode plate being insulated from each other by the separator, the negative electrode plate and the positive electrode plate are spirally wound around a cylindrical core, whereby a cylindrical wound electrode assembly is fabricated. Next, this cylindrical wound electrode assembly is flattened out by a press machine and is formed into a flat wound electrode assembly that can be inserted in a prismatic battery outer can. Subsequently, this flat wound electrode assembly is housed inside the prismatic battery outer can, an electrolyte is then poured therein to fabricate a prismatic nonaqueous electrolyte secondary battery. In some cases, a stacked electrode assembly stacked with a negative electrode plate and a positive electrode plate being insulated from each other by a separator is used. Hereinafter, explanations are given by using the term “flat electrode assembly” including the meaning of both a flat wound electrode assembly and a flat stacked electrode assembly.
When housing the flat electrode assembly fabricated as described above into a prismatic battery outer can made of metal, the flat electrode assembly can contact the edge of the battery outer can and may be damaged. Also, there is a case where electrical insulation is needed between the electrode plate positioned on the outer circumference side of the flat electrode assembly and the prismatic battery outer can made of metal. Thus, in the related art, a protective material or an insulating material has been formed on the outer circumference of the flat electrode assembly so that the surface of the flat electrode assembly is not damaged even if the flat electrode assembly contacts the edge of the battery outer can, and also, so as to secure electrical insulation between the flat electrode assembly and the battery outer can. Here, a related-art prismatic battery including such a protective material or an insulating material will be explained with reference to FIG. 6.
FIG. 6A is an exploded side view of a flat electrode assembly disclosed in JP-A-2007-226989, FIG. 6B is a front view illustrating the flat electrode assembly with the insulating frame attached thereto, and FIG. 6C is a front view illustrating the flat electrode assembly with the insulating frame attached thereto and halfway inserted into a prismatic battery outer can made of metal.
The prismatic battery disclosed in JP-A-2007-226989 includes a flat electrode assembly 83 including a positive electrode substrate exposed portion 81 over which positive electrode binder is not spread at one end in the winding axis direction and a negative electrode substrate exposed portion 82 over which negative electrode binder is not spread at the other. The positive electrode substrate exposed portion 81 and the negative electrode substrate exposed portion 82 are bundled, and the bundled positive electrode substrate exposed portion 81 and negative electrode substrate exposed portion 82 are welded with a positive electrode collector 84 and a negative electrode collector 85, respectively (see FIG. 6A). These positive electrode substrate exposed portion 81, the negative electrode substrate exposed portion 82, the positive electrode collector 84, and the negative electrode collector 85 are covered by a frame 86 having a U-shaped cross section and having a U-shaped outer shape (see FIG. 6B). The flat electrode assembly 83 is housed inside a battery outer can 87
In a prismatic battery 80 disclosed in JP-A-2007-226989, since the bundled positive electrode substrate exposed portion 81 and negative electrode substrate exposed portion 82 are respectively welded with the positive electrode collector 84 and the negative electrode collector 85, electrical resistance between the positive electrode collector 84 and the positive electrode substrate, and between the negative electrode collector 85 and the negative electrode substrate can be made small. Also, electrical insulation between the positive electrode plate, the negative electrode plate, the positive electrode substrate exposed portion 81, and the negative electrode substrate exposed portion 82, and the battery outer can 87 becomes preferable. Thus, the prismatic battery 80 is preferable for EVs and HEVs on which large current is caused to flow, and which is powered by a number of batteries being serially and parallely coupled.
However, as shown in FIG. 7 for example, the positive electrode collector 84 and the negative electrode collector 85 of the prismatic battery 80 are welded to a positive electrode substrate welding portion 81a and a negative electrode substrate welding portion 82a, respectively, that are formed or welded so as to be thinner than the flat electrode assembly 83 with the flat portions of the positive electrode substrate exposed portion 81 and the negative electrode substrate exposed portion 82 bundled.
FIG. 7 is a perspective view of the flat electrode assembly shown in FIG. 6.
Consequently, since the positive electrode collector 84 and the negative electrode collector 85 are respectively positioned closer to the center side than the flat surface of the flat electrode assembly 83, unevenness is generated between the flat surface of the flat electrode assembly 83, and the positive electrode collector 84 and the negative electrode collector 85. Thus, when the flat electrode assembly 83 is inserted in the prismatic battery outer can 87 with the assembly 83 covered with the insulating frame 86 made of an insulating sheet and having a U-shaped outer shape, a gap is generated between the insulating frame 86, and the positive electrode collector 84 and the negative electrode collector 85.
Since batteries for EVs and HEVs are used in places where frequent vibration occurs, if a gap is formed between the insulating frame 86, and the positive electrode collector 84 and the negative electrode collector 85, the flat electrode assembly 83 in the battery outer can 87 is prone to fluctuate, which is not preferable. In order to prevent the flat electrode assembly 83 from vibrating in the battery outer can 87, a projection that serves as a fixing portion may be provided to the positive electrode collector 84 and the negative electrode collector 85, and the projection is made to contact the insulating sheet that forms the insulating frame 86 in the battery outer can 87. However, the related-art projection projects from the flat electrode assembly 83 towards the battery outer can 87 with the amount thicker than the insulating sheet that forms the insulating frame 86. Thus, when the flat electrode assembly 83 is inserted in the prismatic battery outer can 87, even a slight deviation of the center line of the flat electrode assembly 83 with respect to the center line of the battery outer can 87 in the width direction makes the projection of the positive electrode collector 84 or the negative electrode collector 85 penetrate through the insulating sheet of the insulating frame 86. Consequently, a short circuit may occur between the flat electrode assembly 83 and the battery outer can 87.
In order to prevent the projection of the positive electrode collector 84 or the negative electrode collector 85 from penetrating the insulating sheet of the insulating frame 86, the area of the surface of each projection may be made large to make the contacting area between the projection and the insulating sheet large. However, if the contacting area between the projection and the insulating sheet is made large, resistance when inserting the flat electrode assembly 83 with the insulating film 86 into the prismatic battery outer can 87 becomes large. Thus, insertability of the flat electrode assembly 83 to the can is degraded and affects the manufacturing efficiency of the prismatic battery.
In JP-A-2000-150306, as shown in FIG. 8A, an example of a current collecting method of a battery or a capacitor 90 is shown. In a clamping current collecting member 91, a retaining portion 92 is formed by folding the tip of the clamping portion into a U shape. The retaining portion 92 is made to contact both a flat electrode assembly 93 and an inner wall of a case body 94. By the elastic force of the retaining portion 92, clamping of the flat electrode assembly 93 and a collector foil stacked portion 95 is made strong. In this battery or the capacitor 90, if the case body 94 is made of a conductive material, in order to insulate a contacting portion of the retaining portion 92 and the inner wall of the case body 94, the retaining portion 92 is covered with an insulating material, or an inner surface of the case body 94 is covered by an insulating film. Further, as shown in FIG. 8B, the contacting portion on the case body 94 side is provided with a resin projecting piece 96.
FIG. 8A is a cross-sectional view of a collector terminal disclosed in JP-A-2000-150306, and FIG. 8B is a partial enlarged view of a modification of the collector terminal shown in FIG. 8A.
The clamping current collecting member 91 of this battery or capacitor 90 has the clamping of the flat electrode assembly 93 and the collector foil stacked portion 95 made strong by the elasticity of the retaining portion 92 folded in a U shape. Thus, the tip of the retaining portion 92 folded in a U shape largely projects from the flat electrode assembly 93 towards the case body 94 side, and the volumetric efficiency is degraded. Also, large compression stress is applied to the insulating material used between the retaining portion 92 folded into a U shape and the case body 94. Thus, using a thin insulating film as an insulating material is difficult, and this also leads to degradation of the volumetric efficiency. In addition, providing the resin projecting piece 96 as shown in FIG. 8B in the case body 94 not only leads to degradation of the volumetric efficiency, but adversely affects the manufacturing efficiency of batteries or the like. Thus, simply adopting the current collecting method of the battery or the capacitor 90 shown in JP-A-2000-150306 for fixing flat electrode assemblies of the prismatic batteries for EVs or HEVs is difficult.